The beige mouse is an animal model for the Chediak-Higashi syndrome in man. The large granules of the mast cell of that animal make possible detailed analysis of their exocytosis to the plasma membrane. We have developed an instrumental array capable of simultaneous physiologic and anatomic real-time measurements of living cells with control of the internal millieu, first using it to measure the capacitance of secretory and phagocytic peritoneal cells from the beige mouse. We have devised a method for purification of mast cells. Macrophages show larger capacitance than the mast cell, although the macrophage is a smaller cell. This paradox suggests a high degree of membrane folding in the macrophage, a finding consistent with its role in phagocytosis. Large capacitance changes are seen at the instant of fusion in mast cells, consistent with the calculated area of the secretory granule. Swelling of individual granules is also seen. It is expected that simultaneous measurement of swelling and fusion will prove or disprove the osmotic theory of exocytosis.